The New York Stock Exchange has a series of "circuit breakers" in place to calm investors' nerves when they're panicked.
A circuit breaker was tripped Monday morning shortly after trading began.
The S&P 500 fell by more than 7%, halting trading for 15 minutes.
The next circuit breaker would be if the market falls by 13%. That would pause trading for another 15 minutes.
If the market plunged 20%, everyone would go home: Trading would stop for the day.
Circuit breakers pause and ultimately halt trading to avoid a repeat of "Black Monday" on October 19, 1987, when the Dow crashed 22.6% in a single day.